1. Field of the Invention
The present invention is in the field of apparatus for non-destructive, non-contacting ultrasonic measurement of wall thickness of an extruded plastic tubular shape, provided with means for compensating for non-uniform temperature conditions which would otherwise interfere with the accuracy of the ultrasonic testing.
2. Description of the Prior Art
There is a need in the extrusion of plastic layers for forming cables and lines as well as in the extrusion of plastic pipes and the like to measure directly the thickness of the extruded layer or the wall thickness. Ultrasonic measurements based upon the pulse-echo method provides a suitable measuring procedure. Since the measurement is based on the transit time of the sound wave, the speed of sound in the material under test is a factor in the measured result. For plastics such as polyvinyl chloride, polyethylene, and the like, the sound propagation velocity is temperature dependent to a significant degree. This factor is particularly noticeable when utilizing direct measurement in the manufacturing proces itself followed by the extrusion into a cooling path wherein considerable fluctuations from room temperature as well as additional temperature gradients in the layers to be measured may occur.
If measurements are to be taken at the earliest possible time following the extrusion, a temperature compensation or a correction of the measured thickness values is vital for more precise measurements. In the known ultrasonic measurements of wall thickness used in the manufacture of cables, lines, pipes and the like, the measuring systems make no correction of the measured thickness value on the basis of temperature changes. Therefore, only a limited absolute precision can be achieved in these existing measuring systems, particularly when the temperature gradient is not uniform in the wall layer, but has a specific profile.
The influence of relative process parameters on the ultrasonic measurement can be empirically acquired in tables or constructed functions in a test series. For example, a correction of the ultrasonic measurement is possible with the assistance of a microprocessor but is only approximately possible with limited precision.
In U.S. Pat. No. 3,930,404 there is described a system for measuring dimensions of a tubular article. It employs a pair of ultrasonic transducers which are located diametrically opposite the article in a coupling bath. The tubular article is simultaneously rotated and translated between the transducers to generate a helical inspection path. The transducers direct pulses of ultrasonic energy at the outer tube surface for reflection back to the transducers. Some energy enters the tube wall to the inner tube surface before being reflected back. The signals thus received are electronically processed to provide an indication of the inside and outside diameters and the wall thickness.